1. Field of the Invention
The present invention is directed to a process of removing ion-implanted photoresist from a workpiece. More specifically, the present invention is directed to a process of removing hardened photoresist, disposed on a workpiece, that has previously been exposed to ion implant processing, utilizing a composition which includes liquid or supercritical carbon dioxide and an alkanol.
2. Background of the Prior Art
The employment of photoresists in lithographic processes in the fabrication of workpieces, to precisely position transistors, resistors, interconnects and the like thereon, is ubiquitous in the art. Another common processing step in the formation of workpieces is ion implantation which is utilized in the manufacture of large scale integrated circuits. Ion implantation, utilizing ion beams, is employed to implant regions of a workpiece with a controllable level of heterogeneous ionic impurities in order to create wells of either excess positive or negative charge. This doping method affords a precise way of defining electronic band structure and conductivity in the local areas which will ultimately become electronically active parts of a solid state electronic device or circuit.
In defining regions of positive and negative charge, and in conjunction with other integrating processing steps, it is sometimes necessary to carry out implant processing through a layer of masking material or photoresist which may inadvertently become crosslinked, hardened or otherwise chemically or physically changed by the incident ion beam in its passage through the photoresist layer. As a result the xe2x80x9cimplanted photoresistxe2x80x9d material is rendered more difficult to remove by solvent dissolution means and thus presents a problem in workpiece manufacturing.
The aforementioned problem has been identified in the art. Thus, methods are known for removing implanted photoresist which, although effective in removing the photoresist, requires the workpiece to be subjected to long wet chemical processing operations, high temperature and pressure or both. This, in turn, limits the materials that can be utilized as the photoresist subjected to ion implantation.
As stated above, methods of removing resists are known in the art. One such method, disclosed in U.S. Pat. Nos. 5,651,860 and 5,895,272, involves hydrogenating the resist material by immersing that material in pressurized boiling water.
U.S. Pat. No. 5,882,489 sets forth a method of removing a resist layer, particularly in via holes, in surfaces of semiconductor wafers. In this process the resist layer is ashed to form a soluble organic compound which is rinsed off in water. Subsequently, the ashed and rinsed semiconductor device is subjected to sputtering in an etching and stripping chamber to sputter away the resist layer.
U.S. Pat. No. 5,824,604 describes another method of removing photoresist from a substrate which entails little etching of oxide from the substrate surface. In this process the substrate, upon which photoresist is disposed, is contacted with a plasma which includes an oxidizing gas, a halogen-containing compound and a hydrocarbon that releases hydrogen under plasma-forming conditions.
Yet another method, disclosed in U.S. Pat. No. 6,024,887, of stripping a photoresist layer from a substrate, wherein the photoresist layer is specifically directed to an ion implanted photoresist layer, involves treating the ion implanted photoresist layer with a first plasma employing a first etchant gas composition comprising a fluorine-containing species. Thereupon, a second plasma, employing a second etchant gas composition, which includes an oxygen-containing species, results in stripping of the ion implanted photoresist layer from the substrate without plasma-induced damage to the substrate.
U.S. Pat. No. 6,126,772 sets forth still another method of resist removal. In this method an adhesive layer is formed on an article on which a resist is disposed. The adhesive layer may be applied before or subsequent to the deposition of the photoresist layer. It is imperative, however, that the adhesive layer have a modulus of elasticity of at least 1 Kg/mm2, independent of whether the adhesive layer is curable or not. Upon formation of the adhesive layer, that layer is peeled from the article as a unitary sheet thus removing the resist material which remains attached to the adhesive layer.
U.S. Pat. No. 6,203,406 mentions yet still another method of removing photoresist. In this method a high velocity aerosol of at least partially frozen particles is directed at a foreign material, which may be a photoresist, on the surface of a substrate.
The use of liquid or supercritical carbon dioxide in the removal of residue, which may be formed from stripped photoresist, is disclosed in U.S. Pat. No. 5,908,510. However, the use of liquid or supercritical carbon dioxide in the removal of hardened photoresist is not disclosed or suggested by this reference.
International Publication WO 02/15251 teaches the removal of photoresist and photoresist residue from semiconductors using supercritical carbon dioxide. In this process photoresist, on the surface of a semiconductor, is removed by disposing that photoresist-covered semiconductor substrate in a pressure chamber with supercritical carbon dioxide and a stripper chemical in which the maximum pressure is maintained at 6,000 psig.
The disadvantages of the process described in the ""251 publication resides in the requirement that a strong stripper chemical, having detrimental environmental effects, must be employed in the photoresist removal step. Moreover, that process, which introduces environmental dangers, does not address the most difficult to remove photoresists, those which were previously ion-implanted.
The present state of the art, summarized above, strongly suggests the need for a new process which permits simplified photoresist removal consistent with non-detrimental environmental consequences.
A new process for removing photoresist from a workpiece has now been developed which is highly effective and which liberates substantially no environmentally dangerous material into the atmosphere.
In accordance with the present invention a new process is provided which includes the steps of contacting a workpiece, upon which a photoresist, previously exposed to ion etching, is disposed, with a composition which comprises a mixture of liquid or supercritical carbon dioxide and an alkanol, said alkanol present in a concentration of between about 2% and about 20%, said percentages being by volume, based on the total volume of the composition.